Metal acetylacetonates are highly efficient catalysts for a wide variety of organic transformations such as oligomerization, polymerization, hydrogenation, isomerization, coupling etc. They are also used in rubber technology for vulcanization, for extraction and separation of metals, as NMR shift reagents, in microelectronic devices, for synthesis of high quality semiconductor materials for optoelectronic devices, for separation of enantiomers, as a source of metal or metal oxides for controlled deposition, as fungicides, in pigments as color stabilizers, as carbon scavengers for diesel fuels, as combustion control catalysts for rocket fuels, and in laser technology. Metal acetylacetonates, often referred as metal chelates, are well known in the art as witnessed for example by U.S. Pat. Nos. 3,231,597 and 3,291,660.
Reference is made to British Patent 289,493 wherein preparation of metal acetylacetonates was carried out by the reaction of excess of acetylacetone or a solution of a solid salt of it in an inert solvent followed by refluxing the metal oxide, hydroxide, carbonate or basic carbonate of the metal. The disadvantages are the reactions are slow with quite poor yields, requirement of inert organic solvent, energy consumption due to necessity for refluxing, use of excess acacH thereby increasing the costs, chances of contamination, and restricted operability due to solubility problem. Reference is made to J.Chem.Soc., 1938, 1254 and Inorg. Syntheses, 1946, 2, 119 wherein preparation of metal acetylacetonates was carried out in nonaqueous solution by the reaction of metal salt and acetylacetone. The disadvantage here is that this method is applicable to those metal salts only that are soluble in the chosen nonaqueous solvent.
Reference is also made to Compt. Reid., 1943, 157,30 wherein preparation of metal acetylacetonates was carried out by the reaction of acetylacetone with a metal oxide, hydroxide, carbonate or basic carbonate in aqueous solution. The disadvantages are that the reaction is very sluggish, the possibility of contamination with the by-products exists, and excess acacH is used thereby increasing the costs of the process. Reference is made to J.Chem.Soc., 1947, 1084 and J.Am.Chem.Soc., 1948, 70, 3142 wherein metal acetylacetonates were prepared by the reaction of acetylacetone with a metal oxide, hydroxide, carbonate or basic carbonate in aqueous solution and controlling the pH of the solution by gradual addition of a weak base such as ammonia. The disadvantages are that unless care is exercised there is high probability of the end product being contaminated by metal hydroxide or basic salt, the use of buffer leading to the addition of extraneous ion, which in turn may contaminate the product. Addition of ammonia may result in high local concentration causing precipitation of metal hydroxide of basic diketone derivatives.
Reference is made to J.Chem.Soc., 1925, 2379 and J.Org.Chem., 1948, 13, 249 wherein preparation of metal acetylacetonates was carried out in anhydrous inert medium containing the ligand and metal. The disadvantage is that this method is applicable to the synthesis of only active metal derivatives such as alkali metals, alkaline earth metals and the process is expensive since anhydrous solvents are used. Reference is made to Anal.Chem., 1951, 23,174 wherein Mn(acac)3 was prepared by the reduction of MnCl4 by acetylacetone. The disadvantages are requirement of unstable Mn(IV) compound, involvement of an extra preparation step and contamination of the end product by chloride.
Reference is also made to J. Am. Chem. Soc., 1951, 73, 4416 and Inorg. Syntheses, 1966,7,183 wherein Mn(acac)3 was prepared by air or chlorine oxidation of a basic solution of Mn2+ in the presence of acacH, or by KMnO4 oxidation of Mn2+ in the presence of acacH and a large excess of sodium acetate. The disadvantages are the deleterious effect of alkali on the end product, contamination by chloride ions, or by sodium acetate. Reference is made to J.Am.Chem.Soc., 1953, 75, 2446 wherein uranium(VI) (c.f. UO22+) acetylacetonate was prepared by using sodium hydroxide for adjusting the pH conducive to the synthesis. The disadvantages are contamination of the product because of the use of a large quantity of alkali and involvement of extra purification steps. Reference is also made to J.Am.Chem.Soc., 1953, 75, 2736 wherein preparation of metal acetylacetonates was carried out by the reaction of soluble salt of acetylacetone with a soluble salt of metal. The disadvantages are prior preparation of the salt of acetylacetone and reactions being conducted at higher pH causing the formation of by products such as M(acac)X, M(acac)OH and M(acac)3−. Reference is made to Anal.Chem., 1953, 25,881 and 1954, 26, 375 wherein preparation of metal acetylacetonates was carried out by incorporating solvent extraction. An immiscible liquid was added to the reaction mixture to extract the desired metal. The disadvantage is that some of the side products formed are also extracted with the pure product due to their partial solubility. Reference is also made to Inorg. Syntheses, 1957, 5, 105 wherein the metal acetylacetonates were prepared by the reaction of the metal ion and acetylacetone, i.e., by chelation of the metal ion by the bidentate ligand. As a consequence it releases proton decreasing the pH of the reaction mixture. Unless the metal chelate is highly soluble, the reaction between the metal ion and acetylacetone will come to equilibrium short of completion because of the increase in concentration of free acid in the solution. The pH suitable for the successful synthesis of metal acetylacetonates was ascertained to be 5.5. In order to shift the equilibrium to the right acidity generated, as mentioned above, can be controlled by the use of a suitable buffer. For this reason the use of acetate is recommended for such preparations. In some case homogeneous generation of ammonia in the reaction solution can be achieved by adding urea to the solution and heating. The disadvantage is that the direct reaction of acetylacetone and a salt in water is limited by the low solubility of metal acetylacetonate. And there is a definite possibility of contamination of the product by the buffer, ammonia, or acetate.
Reference is made to U.S. Pat. No. 3,946,057 wherein the preparation of metal acetylacetonates was achieved from the reaction of acetylacetone with a metal halide or hydroxide thereof in the presence of alkyne oxide and organic solvents. The disadvantages are contamination by chloride, partial reaction with HCl, expensive alkyne oxide and use of organic solvents. Alkali metal halides have not been found suitable for use in this process. Reference is made to U.S. Pat. No. 4,008,260 wherein the Co (II) acetylacetonate was first made and then reacted with acacH in an organic solvent and 30% H2O2 in higher proportions under reflux. The disadvantages are extra preparation of Co(II) acetylacetonate, use of organic solvents and a high amount of H2O2 and temperature.